The present invention relates to a method for controlling ignition timing of an internal combustion engine for an automobile.
A learning control system for correcting the ignition timing has been proposed. The control system is adapted to advance the ignition timing so as to produce a maximum torque as long as a level of an engine knock does not exceed a tolerable level. If the knock occurs, the control system operates to retard the ignition timing by a predetermined value. A maximum ignition timing for producing the maximum torque without causing the knock due to changing a type of the engine or octane number of fuel used in the engine. Namely, when high-octane gasoline is mixed with low-octane gasoline, the ignition timing must be more advanced than that of the low-octane gasoline for increasing the torque. Since the octane number varies with the quality of the fuel, it is necessary to correct the ignition timing through a learning control system.
The maximum ignition timing is advanced as the octane number of the fuel increases. However, it is not advisable to advance the timing in accordance with an increase of the octane number, and to exceed a limit ignition timing only for producing the maximum torque dependent on engine operating conditions because the torque is reduced.
Japanese Patent Application Laid-Open 61-157768 (U.S. Pat. No. 4,736,723) discloses a learning control system for the ignition timing control. In the system, a memory storing maximum ignition timings and basic ignition timings is provided, arranged in accordance with engine speed and intake passage pressure. The difference between maximum ignition timing and a basic ignition timing derived from the memory is calculated. A correcting value is obtained by multiplying the difference by a coefficient. The ignition timing is obtained by adding the correcting value to the basic ignition timing. However, the calculated ignition timing does not coincide with a theoretically desired maximum timing. For example, in a low engine speed range the timing is advanced from the desired maximum timing, and in a high engine speed range the timing is retarded. Consequently, the ignition timing is not properly controlled so as to produce a maximum torque with selected fuel.